Chemistry Question! Help is much appreciated!

Question

1. What type of reaction starts with two or more compounds and produces one compound?

a. A formation reaction

b. A decomposition reaction

c. A complete combustion reaction

 

2. If a reaction has a ΔH of -120.0 kj/mole and is spontaneous for all temperatures less than 200.0 K, what is its delta S?

Answer 1

1. Reaction types are important:

• A formation reaction (aka synthesis) is a reaction in which simpler bits (elements or compounds) are combined into a larger compound
• A decomposition is the opposite of a synthesis reaction. It takes one large compound and breaks it into smaller bits.

• A combustion is a burn, usually of a hydrocarbon (CxHy), in oxygen to generate CO₂ gas and H₂O vapor

 

Based on the descriptions above, you should be able to figure out which of the three options is you answer.

 

2. The key in solving this problem is in realizing that below 200K the reaction is spontaneous (?G<0) and above 200K the reaction is no longer spontaneous (?G >0). This means that ?G = 0 at 200K.

 

So, we know that at 200K:

 

?G = 0 kJ/mol

?H = -120kJ/mol

T    = 200K

?S  = x

 

Since you have only one unknown, you can solve Gibb's free energy equation for the unknown and plug your know values in, as shown below.

 

?G = ?H – T?S <=> –?S = (?G – ?H)/T <=> ?S = (?H – ?G)/T

 

?S = (-120kJ/mol – 0kJ/mol)/200K = ?S in kJ/molK...

 

it's going to be a bit small, which is why ?S is usually given in J/molK instead of kJ/molK

 

I will let you actually complete the computation.